It has been commercially desirable for many years to have plastic, glass, or other like non-conductive substrates provided with a metal coating or plating on its surface either as a continuous coat or as a patterned or discontinuous coating or plating. In addition, numerous related applications exist in regards to providing a metal coating or plating to composite substrates having both a conductive metal portion and a nonconductive portion, usually plastic. Such composite substrates are commonly comprised of a plastic sheet having a thin metal foil, usually copper, laminated or clad to the two sides of the plastic sheet leaving the non-conductive plastic sandwiched between two metal surfaces. Holes are usually drilled through the metal clad and the plastic, exposing the plastic where the holes are drilled. These composite substrates, after being electroplated, are used to produce printed circuit boards for electrical or electronic applications.
So-called printed circuit boards are produced by variations of two basic systems, one of which is referred to as the additive system and the other the subtractive system. The details of both are well known but, briefly, in the additive system the starting composition is comprised of plastic with no metal foil, and the metal circuit is then built up upon the non-conductive substrate in the desired pattern. In the subtractive method a non-conductive substrate, such as epoxy bonded fiberglass, has adhered to two sides thereof a metal cladding or laminate, most often copper. Holes are drilled through the copper laminate board exposing the plastic. It is then deburred, chemically cleaned and rinsed. The board is then treated with a dilute solution of hydrochloric acid, dipped into a catalyst, mostly commonly a palladium-tin catalyst, to activate the plastic for electroless deposits, rinsed in water, treated with an accelerator (generally fluoroborate based) to remove the tin compound, again rinsed and immersed into an electroless plating bath to electrically connect the two metal (copper) sides by plating the inside of the holes as well as the exposed sides and edges of the board. A plating resist is then applied in the circuit pattern desired. The board is then cleaned, electroplated with copper followed by solder, the resist removed with a solvent to expose the copper that is covered and this copper is removed by etching, thereby providing the desired circuit.
In all of such applications, the non-conductive portions of the substrate must be activated since neither electroless metal plating or electro metal plating can be carried out on the non-conductive portions of the substrate in its absence. The activation is followed by an electroless metal plating of a sufficient nature so that it will carry a current and permit subsequent electroplating.
It is, of course, not commercially feasible to treat such composite substrates to activate or catalyze only the non-conductive portions and as a result, the entire composite substrate is immersed or dipped into an activating solution or colloid. In this manner of processing such composite substrates, the non-conductive portions are not only activated, but the conductive or metal portions thereof are also contacted with the activating solutions or colloids. Unfortunately, the activating solutions coming into contact with the metal conductive portions of the composite substrate may contaminate the metal portions and this contamination seriously interferes with the bond of the subsequent electroless metal deposit which also deposits on the metal portions.